as willies are more sticky and humid; I assume when a man buys a new pair of pants they're a bit like new socks and fluff goes everywhere.
I assume wrong?
In which case, why don't they make socks out of pants material to decrease the fluff output?

is because it's a larger recess, it's pretty dry, and you have hairs 'guiding' stuff towards it.

If you have REALLY fluffy pants and don't shake yourself off properly, then I suppose you might get so fluff there, but for want of a better description, basically the foreskin pushes stuff down and away from your urethral opening.